Python List Slicing with None as argument

Question

Via trial and error I found out that

my_list = range(10)
my_list[:None] == my_list[:]

I use this for django query sets so I can define a si

Answer 1

Your way is fine, but I would prefer :

some_queryset[:length] if length else some_queryset

or

some_queryset[:length] if length else some_queryset[:]

which are readable with less knowledge of how slicing treats these special cases.

Answer 2

There is no difference between using None or using empty slicing like [:] but using None is useful when you want to use it within a list comprehension or use it under a condition for slicing, for example :

>>> [my_list[:length if length%2==0 else None] for length in [1,2,3]]
[[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [0, 1], [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]]

From a comment in CPython source about slice function :

Return a new slice object with the given values. The start, stop, and step parameters are used as the values of the slice object attributes of the same names. Any of the values may be NULL, in which case the None will be used for the corresponding attribute. Return NULL if the new object could not be allocated.

Answer 3

It should be safe. In Python, something[<sliceexpr>] is equivalent to something[slice(...)], and the documentation for the slice type clearly indicates that the arguments for stop and step default to None.

Answer 4

Yes, it is fine to use None, as its behavior is specified by the documentation:

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j. If i or j is greater than len(s), use len(s). If i is omitted or None, use 0. If j is omitted or None, use len(s). If i is greater than or equal to j, the slice is empty.

Using None for one of the slice parameters is the same as omitting it.